1. Field of the Invention
The present invention relates to a charge-coupled device (CCD), and more particularly, to a charge-coupled device which improves the resolution of a device.
2. Discussion of the Related Art
In general, a CCD can be divided into a light-receiving part for receiving light, a transfer channel part for transferring charges, and a charge detector of observing the amount of signal charge and converting it into a voltage.
Recently, as systems utilizing the CCD attempt to achieve a light weight and a high resolution, the number of pixels has increased and the size of a CCD chip has decreased. Thus, the area occupied by the light-receiving part that can receive light has decreased, reducing the signal amount and deteriorating characteristics such as a dynamic range. In order to improve the resolution and to maintain the high sensitivity, the reduction of noise level is necessary. However, since the relation between the sensitivity and resolution is a trade-off, simultaneously improving both is difficult.
When imaging is carried out, the degradation of definition due to the difference of the partial brightness of the image is caused by the limited dynamic range, so that this aspect becomes an important cause of deteriorating device characteristics.
Recently, the Matushita Co. of Japan has suggested a method with the construction of one screen such that a long exposure signal (LE signal) and a short exposure signal (SE signal) appear alternately, thereby enlarging the dynamic range of the CCD.
However, with this method, a method of transferring the image charge should be changed. Further, the problem of deteriorating sensitivity of the device cannot be solved.
A conventional device will how be described with reference to the attached drawings.
FIG. 1 shows the construction of a conventional charge-coupled device. In general, the charge-coupled device comprises a plurality of photodiodes 1 arranged in a matrix for converting an optical image signal into an electrical signal, vertical charge transfer regions 2 transferring the image charge from the photodiode 1 region in a vertical direction, a horizontal charge transfer region 3 for transferring the image charge transferred in the vertical direction through vertical charge transfer region 2 in a horizontal direction, and a sensing amplifier 4 for sensing the image charge transferred in the horizontal direction and outputting a signal to a peripheral circuit.
In the CCD as constructed above, the image charges converted and generated in the photodiodes 1 are moved to vertical charge transfer region 2 by a pulse that is periodically applied to a transfer gate (not shown).
Then, the image charges moved to vertical charge transfer region 2 and horizontal charge transfer region 3 are transferred to an output terminal of the CCD, according to the variation of the potential well caused by a clock signal applied repeatedly.
When a CCD camera is constructed using the above CCD, by detecting an average illumination of an image imaged by the CCD through a lens, an iris should be adjusted so that the average illumination of the above image is fit for the dynamic range of the CCD.
If the signal level is a specific value or less, gain should be properly adjusted through an automatic gain controller.
However, in the conventional CCD, the following problems exist.
In general, the dynamic range of the CCD is not more than 1/10 of a brightness that exists. in nature. Accordingly, imaging one object with the above brightness on one screen by using the charge-coupled device is impossible, because even if the iris is adjusted, imaging is possible only if the imaging screen is clear or dark on the whole. Further, imaging the object in which the clear part and dark part exist partially together is impossible because the degradation of definition occurs in the dark part.